In small-sized radio devices, such as mobile phones, the antenna or antennas are preferably placed inside the cover of the device, and naturally the intention is to make them as small as possible. An internal antenna has usually a planar structure so that it includes a radiating plane and a ground plane below it. There is also a variation of the monopole antenna, in which the ground plane is not below the radiating plane but farther on the side. In both cases, the size of the antenna can be reduced by manufacturing the radiating plane on the surface of a dielectric chip instead of making it air insulated. The higher the permittivity of the material, the smaller the physical size of an antenna element of a certain electric size. The antenna component becomes a chip to be mounted on a circuit board. However, such a reduction of the size of the antenna entails the increase of losses and thus a deterioration of efficiency.
FIG. 1 shows an antenna component and a whole antenna according to application FI 20040892, known by the applicant. The antenna component 100 comprises an elongated and rectangular dielectric substrate 110 and two antenna elements on its surface. The first antenna element 120 comprises a portion 121 partly covering the upper surface of the substrate 110 and a head portion 122 covering one head of the substrate. The second antenna element 130 comprises symmetrically a portion 131 covering the upper surface of the substrate partly and a head portion 132 covering the opposite head. Each head portion 122 and 132 continues slightly on the side of the lower surface of the substrate, thus forming the contact surface of the element for its connection. In the middle of the upper surface between the elements there remains a slot 160, over which the elements have an electromagnetic coupling with each other. The slot 160 extends in the transverse direction perpendicularly from one lateral surface of the substrate to the other. The antenna component 100 is located on the circuit board PCB of a radio device its lower surface against the circuit board. The antenna feed conductor 140 is a strip conductor on the upper surface of the circuit board, and together with the ground plane, or the signal ground GND, and the circuit board material it forms a feed line having a certain impedance. The feed conductor 140 is galvanically coupled to the first antenna element 120 at a certain point of its contact surface. At another point of that contact surface, the first antenna element is galvanically coupled to the ground plane GND. At the opposite end of the substrate, the second antenna element 130 is galvanically coupled at its contact surface to the ground conductor 150, which is an extension of the wider ground plane GND.
At the operating frequency, both antenna elements together with the substrate, each other and the ground plane form a quarter-wave resonator. In compliance with the above described structure, the open ends of the resonators are facing each other, separated by the slot 160, and the electromagnetic coupling is clearly capacitive. The width d of the slot can be dimensioned so that the dielectric losses of the substrate are minimized. The optimum width is in that case e.g. 1.2 mm and a suitable range of variation 0.8-2.0 mm, for example. When a ceramic substrate is used, the structure provides a relatively small size. For example, the dimensions of a component of a Bluetooth antenna operating in the frequency range of 2.4 GHz can be 2×2×7 mm3.
The antenna is tuned by shaping the ground plane and by choosing the width of the slot between the antenna elements. The decreasing the width d of the slot lowers the natural frequency of the antenna. There is no ground plane under the antenna component 100, and on the side of the component the ground plane is at a certain distance s from it. The longer the distance, the lower the natural frequency. In turn, increasing the width d of the slot. The width and length of the ground conductor 150 affect directly the electric length of the second element and thus the natural frequency of the whole antenna, for which reason the ground conductor functions as a tuning element of the antenna. The distance s has an effect also on the antenna impedance, so that the antenna can be matched by finding the optimum distance of the ground plane from the long side of the antenna component.